We show that the magnetic ground state of the double perovskite ${\mathrm{NdSmNiMnO}}_6$ is jointly controlled by $A$-site (Nd/Sm) mixing, $B$-site (Ni/Mn) ordering, and spin-lattice coupling, with the rare-earth sublattice actively participating in the low-energy magnetic balance. Synchrotron x-ray diffraction (SXRD) and Rietveld refinement establish a monoclinic ($P{2}_1/n$) structure with predominant $B$-site rocksalt order and measurable antisite disorder, while x-ray absorption near-edge structure identifies ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ valence states. Magnetometry reveals a ferromagnetic (FM) transition at $T_C\approx 175\mathrm{K}$ driven by ${\mathrm{Ni}}^{2+}\text{-O-}{\mathrm{Mn}}^{4+}$ exchange, followed at lower temperature by a ferrimagneticlike (FiM) regime when the $3d$ and $4f$ sublattices are magnetically ordered. Concomitantly, Raman modes exhibit anomalous hardening below $T_C$ and the refined SXRD lattice parameters display clear anomalies at the magnetic transitions, evidencing pronounced magnetoelastic coupling. Spin-polarized density functional theory captures this competition, yielding nearly degenerate FiM and FM solutions ($\mathrm{\Delta }E\approx 2\mathrm{meV}$ per 20-atom cell) and indicating that coupling of the rare-earth sublattice to the Ni-O-Mn network sets the energy balance. The ferrimagnetic solution carries local moments of $\sim 1.6 {\mu }_{\mathrm{B}}$ on Ni and $\sim 3.0 {\mu }_{\mathrm{B}}$ on Mn and remains insulating, with a gap of 1.6–1.7 eV. With spin-orbit coupling (SOC), the projected ${\mathrm{Sm}}^{3+}$ spin moment is strongly reduced (4.93 to 0.3 ${\mu }_{\mathrm{B}}$), reflecting SOC-driven quenching of the Sm spin polarization, while the gap is nearly unchanged, indicating that the electronic structure is primarily stabilized by Ni/Mn $3d$–O $2p$ hybridization. Our results provide a quantitative framework to disentangle rare-earth $A$-site magnetism from Ni-O-Mn exchange and to rationalize magnetoelastic responses in $A$-site-disordered double perovskites near competing magnetic states.

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• Magnetism
• Magnetoelastic effect
• Ferrimagnets
• Oxides
• Perovskites
• Density functional calculations
• X-ray absorption near-edge spectroscopy
• X-ray diffraction